1. Technical Field
The present invention relates to a light-emitting element, and a light-emitting device, a display device and an electronic apparatus including the light-emitting element.
2. Related Art
An organic electroluminescent element (organic EL element) is a type of light-emitting element and has a structure including a luminescent layer between an anode and a cathode. The luminescent layer includes at least one light-emitting organic layer. In light-emitting elements of this type, electrons and holes are injected to the luminescent layer respectively from the cathode and the anode by applying an electric field between the cathode and the anode, and the electrons and the holes are recombined with each other to form excitons in the luminescent layer. When the excitons return to the ground state, the energy is emitted as light.
In order to enhance the efficiency of hole injection and hole transport in such a light-emitting element, in general, a hole injection layer and a hole transport layer are disposed between the anode and the luminescent layer (for example, Japanese Patent No. 3654909).
In addition, for enhancing the luminous efficiency of the light-emitting element, the levels of the highest occupied molecule orbital (HOMO) and the lowest unoccupied molecule orbital (LUNO) of the hole transport layer are adjusted so that the hole transport layer can block electrons coming from the cathode side (luminescent layer side) to confine electrons and holes in the luminescent layer.
In known light-emitting elements, unfortunately, the hole transport layer cannot sufficiently block electrons, and the electrons gradually enters the hole transport layer in long term use. Consequently, the hole transport layer or the hole injection layer may be degraded by electrons passing therethrough. This problem becomes more pronounced as the driving voltage increases (current density increases), because the ability of the hole transport layer to block electrons is degraded by band bending. It is therefore difficult to increase the lifetime of highly bright light-emitting elements requiring high current density.
In order to enhance the ability to block electrons, it may be effective to use a material having a large energy gap between HOMO and LUMO in the hole transport layer. However, this is not easy in practice because materials that can be used in the hole transport layer are limited.